In digital printing systems that employ multiple printheads to print images on a print media, a system or method for adjusting the registration of the print from the multiple printheads is necessary to ensure that the image planes printed by the individual printheads are in proper alignment with each other. Such a system provides a means to compensate for the stack up of positioning tolerances for the printheads in the printing system. Some registration systems also provide a means to compensate for expansion and contraction of the print media as is passes from one printhead to another within the printing system.
In US Patent Application Publication No. 2011/0074860, multiple image planes are printed by the multiple printheads. Each image plane includes a test mark. Downstream of the printheads, a camera or other image capture device captures an image that includes the test marks from each image plane. The captured image is analyzed to determine the relative position of the test marks. The determined relative positions are compared with the intended relative positions to determine the amount and direction by which the various image planes should be adjusted to yield the proper registration. The appropriate adjustments can be made producing properly registered images. This registration technique, and many others, uses a minimum threshold level of registration. If the misregistration is excessive, some of the test marks may lie outside the field of view of the camera or outside of image processing area of interest in images captured by the camera, or the placement of the test marks within the image may be so far from their intended locations that they lead to ambiguity as to which image plane a test mark is associated with.
There is a need therefore for an improved technique for registering the various image planes printed by a digital printing system in the in-track direction (the direction of motion of the print media) of media travel and in the crosstrack direction (across the width of the print media) of media travel.